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RNAi Delivery Proves Vexing

Emerging Field Simultaneously Holds Both Promise and Problems

RNA interference (RNAi), an exciting yet evolving gene-silencing technology, holds great promise for drug discovery and as a novel therapeutic approach. But troubling doubts have recently emerged as to how quickly that promise will deliver. Perhaps ominously, in November, Roche terminated its multimillion-dollar RNAi therapeutics programs.

One of the main bottlenecks vexing the field is functional delivery. Select Biosciences recent “RNAi Asia” conference discussed the latest developments and innovations, such as development of specialized nanoparticles for delivery, improvements in high-throughput screening, and new in vivo studies.

Delivery problems could be overcome with the use of specialized nanoparticles that entrap small interfering RNAs (siRNAs), suggested Andrew D. Miller, Ph.D., professor at King’s College London. He has an optimistic point of view about RNAi therapeutics.

“RNAi should be the closest thing to a magic bullet for the biopharmaceutical industry,” he said. “In principle, one can get high-quality knockdown with very few side effects or off-target effects. The problem is that RNAi effectors need to be delivered to the target site in a potent form using the smallest possible amount of material. At present, doses are too high. That problem can only be solved with better delivery technologies and improved methodologies.”

A possible solution envisioned by Dr. Miller is the use of cationic, lipid-based, self-assembly nanoparticles that entrap siRNAs into lipoplex nanoparticles, which themselves need to be further stabilized.

“Stabilized nanoparticles with improved circulation half-lives are realized using a polyethylene glycol (PEG) polymer layer to shield the nanoparticle surface. The best stabilized nanoparticles should then be those that are stable until triggered to release entrapped siRNAs by local changes in condition such as pH, or through an external trigger such as from a laser,” he said.

“Our approach is to use a Lego building block-like process to derive stabilized nanoparticle delivery systems in which purpose-designed components are allowed to self-assemble. We have designed RNAi delivery systems according to this paradigm and successfully formulated specialized nanoparticles for the delivery of siRNAs to in vivo targets such as liver and tumors. These are tailor-made delivery solutions.”

The nanoparticle systems are composed of nucleic acids condensed within functional concentric layers of lipid components, surrounded by a biocompatibility layer and equipped with an optional biological recognition/targeting layer, he explained.

There are still significant hurdles to be overcome. “We have achieved proof-of-concept for functional delivery in vivo, but not yet proof-of-therapy,” Dr. Miller said. “We still need time to make significant improvements to the delivery process. We believe that we may be facing a situation in general in which academics still need to solve basic delivery issues before biopharma finds RNAi as attractive as it did several years ago. Delivery will work eventually, but it is likely to take more time and effort than is commonly appreciated,” he concluded.

Progress in Cancer Treatments

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SiRNA treatment eliminates tumor burden in mouse models. Mouse tumors treated with bifunctional siRNA (bottom two mice) do not develop while those in untreated controls (top) do. [University of Queensland]

RNAi for cancer treatment relies on the specific downregulation of target genes that support cancer cells. A number of issues remain, however.

“Effective RNAi therapy requires delivery to each and every cancerous cell in order to directly and specifically eliminate the cancer,” reported Nigel A. J. McMillan, Ph.D., associate professor, principal research fellow and deputy director, Diamantina Institute, University of Queensland. “However, this is unlikely with today’s technology. Currently, only a handful of studies have shown in vivo efficacy in siRNA delivery using vehicles such as cationic lipids, nanoparticles, collagen, and others.”

Dr. McMillan said that he has developed simple, easily prepared lipid particles that he has utilized in animal studies. “Now, we are looking at the immune response after delivery of these particles. We want to know what kind of inflammatory response occurs and if one can unmask an antigen and use that to attack the tumor,” he explained.

“We took a nonbiased approach asking how to silence genes as well as to activate the innate immune response. Certain immune cells can sense and recognize siRNAs in a sequence-specific manner. The systemic delivery of siRNAs will likely recruit immune cells. Although immune activation is of concern, it may also be therapeutically beneficial to enhance the immune response,” according to Dr. McMillan.

“The use of bi- and tri-functional siRNAs—that is, siRNAs that target a gene, activate innate immunity, and/or unmask antigens—allows one to create siRNAs that can combine gene silencing and immunostimulation. This represents a potentially powerful two-pronged attack to kill cancers,” he asserted.

“RNAi doesn’t work like most people think it works,” Dr. McMillan concluded. “We need to more fully understand exactly how it interacts with the immune system before we can realistically target the clinic.”

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